Ceramic materials containing certain mixed metal oxide compositions possess both oxygen ion conductivity and electronic conductivity at elevated temperatures. These materials, known in the art as mixed conducting metal oxides, may be used in applications including gas separation membranes and membrane oxidation reactors. These ceramic membranes are made of selected mixed metal oxide compositions and have been described as ion transport membranes (ITM). A characteristic property of these materials is that their oxygen stoichiometry is a thermodynamic function of temperature and oxygen partial pressure wherein the equilibrium oxygen stoichiometry decreases with increasing temperature and with decreasing oxygen partial pressure.
It is known that the dimensions of materials change with changing temperature due to thermal expansion and contraction. In addition to these thermal dimensional changes, mixed conducting metal oxide materials undergo chemical dimensional changes that are functions of the metal oxide oxygen stoichiometry. At isothermal conditions, an article made of mixed conducting metal oxide material will increase in dimensions with decreasing oxygen stoichiometry. At isothermal conditions, the oxygen stoichiometry decreases with decreasing oxygen partial pressure. Since the equilibrium oxygen stoichiometry increases with decreasing temperature, an article made of mixed conducting metal oxides will contract due to both thermal and chemical dimensional changes as the temperature decreases at a constant oxygen partial pressure. Conversely, an article made of mixed conducting metal oxides will expand by both thermal and chemical dimensional changes as the temperature increases at a constant oxygen partial pressure. This is described in an article entitled “Chemical Expansivity of Electrochemical Ceramics” by S. B. Adler in J. Am. Ceram. Soc. 84 (9) 2117-19 (2001).
Dimensional changes therefore result from equilibrium oxygen stoichiometry changes in mixed conducting metal oxide materials. Changing the temperature at a constant oxygen partial pressure or changing the oxygen partial pressure at a constant temperature will change the equilibrium oxygen stoichiometry of the mixed conducting metal oxide material. When a mixed conducting metal oxide is used as an ion transport membrane, for example, an oxygen partial pressure difference across the membrane creates a difference in the equilibrium oxygen stoichiometry at each of the two surfaces of the membrane, which in turn creates the thermodynamic driving force for oxygen ions to diffuse through the membrane.
During startup or shutdown of a gas separation system using mixed conducting metal oxide membranes, the temperature is increased or decreased and the oxygen partial pressure on one or both sides of the membrane may change. The equilibrium oxygen stoichiometry of the membrane material will change in response to the changes in temperature and oxygen partial pressure. Oxygen anions will diffuse into or out of the membrane material and the membrane material will approach its equilibrium oxygen stoichiometry value. As the oxygen stoichiometry and temperature changes, the dimension of the membrane will change. The time required for the membrane to reach chemical equilibrium with the oxygen partial pressures on the surfaces of the membrane will depend on the oxygen anion diffusion rate into or out of the membrane. The time required for equilibration to occur is a function of the material composition, the temperature, and the dimensions of the membrane modules.
Different membrane compositions will have different oxygen anion diffusivities, and compositions with higher diffusivities will equilibrate with the gas phase faster, all other factors being equal. For a given membrane composition, the oxygen anion diffusivity increases exponentially with temperature. Therefore, equilibration times decrease with increasing temperature. Finally, the equilibration time increases approximately with the square of the characteristic dimension (e.g., length or thickness) of the parts in the membrane modules. Therefore, thinner parts will equilibrate faster than thicker parts, all other factors being equal. As the thickness of a part increases and as the temperature decreases, it becomes increasingly difficult to keep the interior of the part in equilibrium with the gas phase due to sluggish diffusion of oxygen anions into or out of the part.
It is known that temperature gradients in a mixed conducting metal oxide ceramic part can create differential strains due to differential thermal expansion and contraction. Similarly, oxygen stoichiometry gradients in a ceramic part can create differential strains due to differential chemical expansion and contraction. This gradient in oxygen stoichiometry may be sufficiently large to create a correspondingly large differential chemical expansion, and therefore large mechanical stresses, that lead to failure of the part. Therefore, it is desirable to avoid differential chemical expansion or at least to control the differential chemical expansion to below maximum allowable values.
There is a need in applications of mixed conducting metal oxide ceramics for methods to heat or cool ceramic articles such as membranes at faster rates without producing unacceptable stresses in the articles. Also, there is a need to determine maximum allowable rates of change in oxygen partial pressures at essentially constant temperatures in order to avoid unacceptable stresses in the articles. However, few solutions have been proposed to solve these problems to date. In one approach, U.S. Pat. No. 5,911,860 discloses the use of composite membranes containing mechanically enhancing constituents such as metals to improve the mechanical properties of mixed conducting metal oxide membranes. Membranes are disclosed that have a matrix material which conducts at least one type of ion, preferably oxygen ions, and at least one constituent which is physically distinct from the matrix material and which enhances the mechanical properties, the catalytic properties, and/or the sintering behavior of the matrix material. The constituent is present in a manner which precludes continuous electronic conductivity through the constituent across the membrane. In a preferred embodiment the matrix material is a mixed conductor which exhibits both electronic and oxygen ion conductivity. The constituent preferably is a metal such as silver, palladium, or a mixture thereof. In other embodiments, the constituent is a ceramic or other electrically nonconductive material. These proposed membrane compositions thus have mechanical properties that allow faster heating and cooling than membrane compositions previously known in the art.
In an article entitled “Prospects and Problems of Dense Oxygen Permeable Membranes”, Catalysis Today 56, (2000) 283-295, P. V. Hendricksen et al describe the problem of mechanical failure of mixed conductor membranes under oxygen partial pressure gradients at steady state operating conditions. It is disclosed that oxygen partial pressure gradients will produce differential chemical expansion that can lead to mechanical failure of the membrane. It is proposed that surface kinetic resistances will decrease the maximum tensile stress in a membrane, especially as the membrane thickness is decreased. Therefore, using thin membranes that have surface kinetic resistances may reduce the maximum tensile stress. However, while the surface kinetic resistances may reduce the maximum tensile stress, the surface kinetic resistances will also decrease the oxygen flux obtained from the membrane, and this in turn would increase the membrane area required for a given oxygen production rate and hence decrease the economic benefit of the membrane process.
U.S. Pat. No. 5,725,965 teaches the use of functionally gradient, compositionally layered, solid state electrolytes and membranes to prevent chemical reduction of membrane layers during operation. This layered membrane structure may reduce the differential chemical expansion during steady state operation but does not address the problem of chemical dimensional changes caused by heating or cooling of the membrane structure.
There is a need in the art for improved methods to reduce the potential for mechanical damage due to dimensional changes during the heating and cooling of articles and systems fabricated from mixed conducting metal oxide materials, particularly in the operation of membrane gas separation and reactor systems under transient values of temperature, pressure, and gas composition. There also is a need for methods to control the differential strain across the membranes in a membrane module when oxygen partial pressures in the module are changed after the module has been heated to an elevated temperature. These needs are addressed by embodiments of the invention disclosed below and defined by the claims that follow.